The results of previous due diligence activities indicate that the site had operated as an industrial facility since the early 1950s with on-site use and storage of hazardous materials. Soil analytical data for the site indicated the presence of hazardous compounds beneath a portion of the industrial complex at concentrations that could result in an unacceptable indoor air inhalation exposure. While not liable to remediate the contaminants, the current owner elected to take preventative measures to preclude unacceptable vapor intrusion exposures in the structure.
Initially, Envirologic installed Vapor Pins® across the impacted structure and collected soil gas samples to evaluate the sub-slab conditions. Sample results indicated the presence of chlorinated compounds in excess of the 2013 Vapor Intrusion Screening Levels (VISLs) in soil gas across an area of approximately 8,500 square feet (SF) of the 47,500 SF structure.
Envirologic conducted pressure field extension testing (PFE) at several locations in the impacted area to evaluate sub-slab depressurization (SSD) as a technology to mitigate the vapor intrusion concerns. Initial test results indicated that the base course materials directly beneath the concrete slab were too tight and/or compacted for effective depressurization. Depressurization from suction pits in the base course materials would have required a considerable blower system or a typical (more economical) blower system with more than 15 suction points.
Additional site evaluation indicated that a lower sand layer (2 "3 feet bgl) was conducive to vapor extraction, and the effects of depressurization of the sand layer extended upward through the overlying soils and base course materials, resulting in a suitable pressure field below the concrete slab.
Based on the results from the PFE testing, Envirologic designed an SSD system that included three suction (vapor extraction) points across the impacted portion of the facility. The suction points were constructed with 0.20 slot well screens positioned to intersect the underlying sand layer. Three-inch-diameter suction piping was extended from each vapor extraction point through a sub-surface trench to a piping manifold along the outer wall of the structure, and then to an OBAR SOE-76 Compact Radial Blower mounted on the exterior wall. Performance monitoring results from vapor pin and test point locations in the impacted area indicated that the system had induced a sub-slab vacuum (> -0.02" water column - WC) across the impacted area.
Following the rescission of the 2013 VISLs and introduction of Volatilization to Indoor Air Criteria (VIAC), it became necessary to expand the mitigation system. Subsequently, Envirologic installed six additional suction points across the facility and installed an additional fan (an OBAR GBR 89 HA Compact Radial Blower) to expand depressurization beneath the entire 47,500 SF structure. The system was completed with visible gauges to alert occupants and the designated system operator of a malfunction; the system was designed to operate continuously with limited inspections or maintenance.
The blower systems are currently operating at a pre-fan vacuum between 9 to 14 inches of WC and flow rates ranging from 60 to 80 cubic feet per minute per fan. Performance monitoring results indicate that the mitigation system has generated a negative pressure in excess of the industry standard sub-slab vacuum of a -0.02 WC across the footprint of the structure. The results of effluent testing indicated that the off-gas meets discharge limitations of Michigan's Air Pollution Control Rules; thus, off gas treatment is not required.
Performance monitoring results indicated a significant decrease in contaminant concentrations in soil gas beneath the structure. At the request of the EGLE, Envirologic completed an addendum to the initial Documentation of Due Care Compliance Report for the facility inclusive of system installation and performance documentation, and monitoring plan with schedules for inspections and required actions. Crown Industrial has since taken control and is performing all operation and maintenance of the system.